The debate format, which included both one-on-one exchanges and debates among groups of three, allowed candidates such as Marc Garneau and Martha Hall Findlay to challenge front-runner Justin Trudeau.

Findlay, a former MP in the Toronto riding of Willowdale, attacked Trudeau for his defence of the middle class by saying he came from a family of privilege.

"You keep referring to the middle class but you yourself have admitted you don't belong to the middle class. I find it a little challenging to understand how you would understand the challenges facing real Canadians," Findlay said to Trudeau.

Her remarks elicited a negative reaction from the crowd but Findlay plowed through the booing to ask: "When did Canada become a society of class?"

"Your campaign has brought a concept into this conversation that I think we need to get beyond … it is equality of opportunity that we should be looking for," Findlay said.

Trudeau allowed Findlay to finish but quickly defended himself and his right to speak up for the middle class.

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"From the very first day I chose to put my name forward to run for a contested nomination in the riding of Papineau, I got people coming at me saying, 'What do you know about representing one of the most economically-disadvantaged ridings in the country,'" said Trudeau.

"I've been lucky in my life to have been given an opportunity to go to great schools, to travel around the world, and what is important for me is to put everything that I have received … in service of my community. And that is what my identity is all about," said Trudeau to a cheering crowd

Up against the clock, Deborah Coyne, who was also part of that segment's three-person panel, simply said, "I'm not going to contribute to that exchange."

Marc Garneau, left, was the first to challenge Justin Trudeau, right, in the third of five Liberal leadership debates in Mississauga, Ont., on Saturday. (Chris Young/The Canadian Press)

Moments later, during another group of three, Garneau drew applause from the crowd for defending Trudeau.

"Let's not be petty Canada. Anybody in this country who wants to run for the leadership of this country should not be looked down upon in any way, whatsoever, regardless of their financial income."

Garneau challenges Trudeau

Moments before the debate officially got underway, Trudeau joked, "I'm eager to find out what Marc's [Garneau] first question is going to be." The two were paired for the first of a series of one-on-one debates.

Garneau asked Trudeau, "What in your résumé qualifies you to be the leader of the country?"

Trudeau responded by saying "leadership is about drawing people in, and about involving Canadians in the kinds of conversations we have, and you can't lead from a podium in a press conference, you can't win over Canadians with a five-point plan, you have to connect with them. And we have to make room for Canadians in the debate that we have coming forward."

That response did not appear to satisfy Garneau, who came back by saying, "If we're talking policy, you haven't really addressed specific policy to the middle class. You've spoken in generalities."

Garneau asked the question again slightly differently. "You and I are both competing for a job … what is it in your résumé that qualifies you to be the future prime minister of Canada?"

"I fought extremely hard to earn a nomination in Papineau that involved going up against strong Liberals and convincing people to come together under my leadership," said Trudeau, who won the Quebec riding in 2008 from the separatist Bloc Québécois.

Garneau was not satisfied with that answer either and while he tried to ask the question a third time, the three-minute clock for their one-on-one debate ran out.

Speaking to reporters after the debate, Garneau said Trudeau was still "short on details" but that ultimately it would be up to voters to decide.

Divisions emerge

The nine candidates vying for the party's top job are MPs Garneau, Trudeau, and Joyce Murray, former MPs Findlay and Martin Cauchon, lawyers Deborah Coyne, David Bertschi, and George Takach, and retired Lt.-Col. Karen McCrimmon.

The themes of the debate ranged from the role of the federal government within the federation, the future of the middle class, transit and infrastructure, job creation, immigration, international trade, and reconnecting with Canadian voters.

The debate was moderated by Cornwall city councillor and former Liberal candidate Bernadette Clément.

Unlike the first two debates, there were no opening or closing statements. Each of the one-on-one segments lasted three minutes.

Takach asked Murray how her plan for co-operation with the NDP or Greens would impact Canadian companies doing business abroad.

Murray replied, "You're talking to a businesswoman here … with a company that has operations in five countries."

"Joyce, I've worked with a lot of start-ups, they don't just plant trees," Takach said.

Coyne, who is the mother of Trudeau's half-sister, was also paired with Trudeau during a one-on-one segment. She asked him whether he would reopen the Canada-Quebec immigration accord and fix the imbalance in settlement funding.

Trudeau said there are different regions with different needs and the party needs to be encouraging all of them to "figure out what works for their communities and their province."

After the one-on-one debates, the candidates took questions from the audience and then debated in groups of three for about five minutes.

Trudeau came face-to-face with Garneau a second time when the two were matched up with Takach but it wasn't the front-runner the lawyer went after.

The three were asked how they would create jobs to reduce the high level of unemployment among youth.

Garneau talked about his plan to delay the repayment of student debt.

Takach accused Garneau of not being able to do math saying, "It's not just youth unemployment, it's youth underemployment. You add those two numbers together and you get to 26 per cent."

After the debate, lawyer Bertschi, who was paired up with Trudeau in the first hour of the debate, was asked why he did not challenge the front-runner.

Bertschi said he didn't attack him because Trudeau will be key in the rebuilding of the Liberal Party.

Coyne, Murray, Cauchon and McCrimmon told reporters after the debate that they have no intention of dropping out of the race despite facing fundraising challenges.

Trudeau asked to state priorities

Earlier in the week, Garneau threw down the gauntlet when he called a press conference in Ottawa calling on Trudeau to clearly state his priorities.

"As Liberals we can not afford to wait until after the leadership race to find out what we signed up for," Garneau said on Wednesday.

"Justin has stated that now is not the time to tell Liberals, to tell Canadians, where he stands and what his plan is for the country. He has said he will do this after the leadership race, sometime before the next election in 2015. In my opinion, this is the same as asking Canadians to buy a new car without test-driving it.

"He has told Canadians that we need a bold plan and a clear vision without defining either. On Justin's two clear priorities, the middle class and youth engagement he has said nothing," Garneau said.

However, Trudeau has shared his position on certain issues.

For instance, during his first national English broadcast interview after entering the leadership race, Trudeau told CBC Radio's The House on Dec. 1 that he is against the proposed Northern Gateway pipeline, that he's in favour of decriminalizing marijuana, that he would reverse the federal government's decision to raise the age of eligibility from 65 to 67, and that he would not raise the GST.

The fourth debate will be held in Halifax on March 3. That day is also the deadline for Canadians interested in signing up to become Liberal supporters so they can vote for the party's next leader.